The present invention is related to method and apparatus for determining nuclear magnetic resonance (NMR) testing apparatus, and more particularly to an improved method and apparatus for spinning sample capsules at a high speed for NMR.
Nuclear magnetic resonance (NMR) is a technique used to determine the characteristics of a sample and to identify or compare molecular structures and compositions that are unique to the material being tested for purposes of identification or analysis. When materials are subjected to electromagnetic radiation of the correct frequency and orientation, they respond by emitting electromagnetic radiation in return from the nuclei contained in the sample, and the characteristics of the electromagnetic radiation emmitted are unique to the particular material. Therefore, if the characteristics of the emitted radiation can be detected with sufficient accuracy, then the material can be fingerprinted by such characteristics for identification, comparison or analysis.
It is also known that if NMR measurements on immobile (e.g., solid) samples are performed while spinning the material at the "magic angle" of 54.7 degrees to a strong magnetic field, the detail and accuracy of the characteristics detected may be enhanced significantly. However, it is sometimes necessary to spin solid material samples at extremely high rates of speed to obtain high resolution resonance lines that indicate the unique characteristics of the material. Some samples require spinning in the range of up to 5,000 to 8,000 hertz (300,000 to 480,000 R.P.M.) to obtain usable results. Therefore, nuclear magnetic resonance (NMR) methods and apparatus are directed to positioning a sample in a strong magnetic field and spinning it about a longitudinal axis, usually, but not always, oriented at the "magic angle" of 54.7 degrees to the magnetic field while alternately irradiating the sample with suitable electromagnetic radiation and receiving the resonant electromagnetic radiation emitted from the sample.
Drawing on the knowledge and experience obtained from high speed spinning apparatus used in ultra-centrifuges driven by jet air streams and utilizing air bearings to minimize friction, a number of NMR apparatus have been developed. A major problem in the field of NMR apparatus is the requirement that an antenna must be positioned as close as possible to the rotating material for transmitting and receiving electromagnetic radiation. While a number of such devices have been developed and used with satisfactory results prior to the development of this invention, all of the prior art devices encountered one or more of a number of persisting problems. Due to the unique requirements of very high speed spinning while maintaining precise orientation to the magic angle in the presence of a closely spaced antenna coil around the sample, any variation from the spinning angle results in deterioration of results. Most of the prior art NMR spinners utilize gas jets to provide the driving force and some type of air bearing to provide support and stability. The most common of such devices utilize the Andrew-Beams structure of a conical axial end bearing, derived from earlier developments in ultra-centrifuge design, on the Lowe-Norberg types of transverse air jets and bearings. However, in order to achieve the required combination of speed, stability, and proximity of antenna components, major components of the prior art apparatus, such as the antenna structure, air drive, or air bearing structure must be disassembled when a sample material is positioned in the testing apparatus or removed from the testing apparatus. Also, particularly in the transverse type prior art NMR spinners, impractically high tolerances of machined components are necessary for the apparatus to maintain precise magic angle or other desired angle at all speeds of spinning. Therefore, these devices suffer from the requirement of impractically high spacial tolerances in manufacturing and machining as well as the necessity of rigid, exotic, and difficult to manufacture materials for container capsules and rotors in order to function. The extremely high tolerances and close clearances of components required in these devices leads to additional problems. For example, in such prior art devices, excessive gaseous fluid drive pressures are required to spin the components at very high speeds. Such high operating pressures can cause a variety of difficulties, such as gas leaks and liquification of the drive fluid in the spinning drive mechanism. Since some materials require NMR testing at extremely cold temperatures for high resolution test results, this problem of liquification of the drive the fluid can inhibit use of desirable cryogenic gases, such as nitrogen, necessary or desireable to conduct extremely cold temperature testing.
In general, the conical type spinner apparatus have the advantage of less rigid tolerances and better capabilities of reaching high speeds with low gas drive pressures, but they suffer from lack of angle accuracy. They also require dissasembly of the components to remove samples due to enlarged conical structures heretofore desired necessary for approaching accurate magic angle maintenance. On the other hand, the transverse type spinner apparatus have the advantages of accurate angle maintenance, but they suffer from impractically close tolerances, expensive materials, difficulty of manufacture, high operating pressures, extreme care required in packing the sample material, and disassembly to remove samples.